Generally, 3-dimensional image display refers to a technology of adding depth information to a two-dimensional image and using this depth information to allow the viewer to feel a sense of 3-dimensional vividness and reality. The 3-dimensional image display technologies are applied in various sectors, particularly in education, health, military, special purpose sectors, etc. Several types, in a variety of forms and methods, according to such technologies have been proposed for typical 3-dimensional image display devices in prior art. Until now, most of these technologies display 3-dimensional images using the principle of binocular disparity of a human being. As there are slight deviations between images presented to the left eye and to the right eye, perception of the disparity by the left and right eyes creates a sense of 3-dimensionality, so that a sense of protrusion may be obtained.
A typical form of prior art is to separate the left and right images, mainly with or without using eyeglasses. Glasses are used in the anaglyph type, polarized glasses type, and liquid crystal shutter type, while glasses are not used in the lenticular sheet type, parallax barrier type, and optical plate type.
Among these conventional technologies, the polarized glasses type is the oldest and most stable 3D display type, and is most widely used in 3D movies and 3D monitors, etc. The biggest drawback of this method, however, lies in the requirement of using special polarized glasses for 3-dimensional images. Thus, it increases eyestrains while wearing special polarized glasses.
The lenticular sheet type and parallax barrier type, among the types not using eyeglasses in prior art, provide low brightness and low resolution images and entail a fixed viewing position for a viewer, causing headaches or dizziness when viewing for an extended period of time.
There are also complete 3-dimensional types, including the holographic and volumetric 3D display types. While these types can produce 3-dimensional images freely in a space, they require expensive laser and precision optical components to display even a still image, and cannot provide real-time 3-dimensional images.
Proposed to solve these problems are some non-glasses types, which utilize reflectors, conventional optical lenses, and concave mirrors, etc., to enable real-time 3-dimensional images at lower costs. However, most of these methods experience distortion of images due to the concave mirrors, etc., and high costs of manufacturing when large devices are used. In particular, when large devices are used in order to obtain 3-dimensional images of a large display, there is a need to form a very large width of space, hindering the utility and applicability of these types.
In addition to these methods using concave minors and reflectors, methods using Fresnel lenses have been proposed in various types for a long time. It has been disclosed that two Fresnel lenses can be used to result in a 3-dimensional image effect, and that one or more Fresnel lenses and reflectors, etc., can be used to create 3-dimensional images in a 3-dimensional image effect. The drawback to these technologies, however, is that the ratio of display to a usable portion is low due to the rendition of object content at a time. In order to obtain 3-dimensional images of a large display, there is a need to form two or more of large transmissive type reflectors and image sources for a large display. Thus, high costs of manufacturing are inevitable when a very large width of space is formed.
In particular, 3-dimensional images, having wide viewing angles enlarged by a liquid crystal projector in which two Fresnel lenses are used to create 3-dimensional images of a large display, can be obtained. The 3-dimensional images generated here may not have distortion of images in viewing angles within the range of 10 to 20 degrees from the center, but may have serious distortion of images in viewing angles beyond the range. Namely, the problem is that distortion of images, in which the 3-dimensional images having an equal sense of depth at the center of a screen become smaller towards the reverse side of the screen on the left and right of the critical angle, can occur.
The problem of embodying 3-dimensional images according to prior art is that 3-dimensional images may not be completely viewed due to the serious distortion of images at the remaining portions except the portion of 10 to 20 degree from the center of the screen. Also, various applications for rendering 3-dimensional images for a large display can be limited due to various problems so that efficient rendering of 3-dimensional images can be difficult. Technologies for creating 3-dimensional images using three Fresnel lenses can be also limited in rendition and applications due to the distortion of images at the remaining portions except the center. The technology using a Fresnel lens and a reflector has merits that are the same as using two Fresnel lenses. In order to obtain 3-dimensional images of a large display, when large devices are used, relatively very large widths of space in the upper and lower sides and the front and rear sides are needed in proportion to the size of the display for 3-dimensional image, and the projection distance of 3-dimensional images becomes shorter by a reflector so that the sense of depth is reduced.
Referring to FIG. 1, a principle for and problems of creating 3-dimensional images using two flat Fresnel lenses in accordance with prior art are described below.
FIG. 1 illustrates a method of rendering 3-dimensional images based on prior art. A flat Fresnel lens has a constant focal length f and is comprised of a first Fresnel lens 115, upon which an input image source is incident, and a second Fresnel lens 120 for projecting 3-dimensional images. The 3-dimensional images, having a different sense of depth in forms of expansion and reduction for the input image source corresponding to a distance d1 from the center of an image source supply part 110 to the first Fresnel lens 115 and a distance d2 from the first Fresnel lens 115 to the second Fresnel lens 120, can be generated at a location separated by d3 from the second Fresnel lens 120.
As shown in FIG. 1, the 3-dimensional images can be generated in the air over a location d3 according to a certain location d1 from the image source supply part 110 by combining the first Fresnel lens 115 and the second Fresnel lens 120, and the 3-dimensional images can be observed by a viewer from a certain area θ1. Here, the distance d2 from the first Fresnel lens 115 to the second Fresnel lens 120 maintains a range in which there is no distortion or aberration of images.
When two Fresnel lenses 115 and 120, with a focal length f1 of the first Fresnel lenses 115 and a focal length f2 of the second Fresnel lenses 120, are arranged in a row within a certain distance, the two Fresnel lenses 115 and 120 can form a focal length f3 127. Here, f3 is a focal length formed by the two Fresnel lenses. Here, when projecting a 2-dimensional image source using two Fresnel lenses 115 and 120, an output image focal plane 125, on which an image is formed, can be formed in a spherical shape inwards from a focal length 127 of the two Fresnel lenses 115 and 120. Meanwhile, when observed from the perspective of the viewer, the output portion of the 3-dimensional images recognized by human eyes can be within the range of viewing angles θ1 having a certain aperture and passing the focal length 127 formed by arranging the two Fresnel lenses 115 and 120. Within this range, the viewer can recognize the same image as the one formed on the output image focal plane 125. Here, the generated 3-dimensional image seem as if it is floating in space. The principle will be described in more detail below. A 2-dimensional image projected from the image source supply part 110 of FIG. 1 can be projected towards the first Fresnel lens 115. The first Fresnel lens 115 and the second Fresnel lens 120 work in combination like a single lens. The output image focal plane 125 is located within the range of the focal length 127 of the two Fresnel lenses 115 and 120. Referring to FIG. 1, the desired 3-dimensional images can be obtained on the output image focal plane 125 only if an image on the screen projected from the image source supply part 110 and the first Fresnel lens 115 maintains a certain distance d1. Also, the 3-dimensional images, formed by the structure of double Fresnel lenses, can form the output image focal plane 125 that has different forms according to the way the directions of grooves of the first Fresnel lens 115 and the second Fresnel lens 120 are arranged. This is because a ray of light refracts to different refracting angles according to the difference of incident angles from the groove plane of the Fresnel lens. The problem is that the output image focal plane 125 formed in a spherical shape is formed on only about ¼ of the center portion against the total display area of the second Fresnel lens 120, and that distortion of image occurs while a 3-dimensional image becomes smaller from the center of the output image focal plane 125 to the edges. The spherical shape output image focal plane 125, which is formed in case the grooves of the Fresnel lenses 115 and 120 face each other, can be formed on a screen 210 (in FIG. 2) of the 3-dimensional image source having a circular boundary plane 225 (in FIG. 2). Accordingly, in comparison with the total size of the screen 210 (in FIG. 2), the display area of the generated 3-dimensional image can be rendered within the circular boundary plane 225 at the center, and in comparison with the total size of the screen 210 as a background against the 3-dimensional image, the display area can be only a smaller portion, so that a sense of depth can be reduced on the whole when creating the 3-dimensional image. Moreover, when observed from a distance to the left and right of the screen, distortion of image occurs due to a spherical surface of the 3-dimensional image.
FIG. 2 illustrates the form of a 3-dimensional image source and viewing angles according to prior art.
Referring to FIG. 2, when crating a 3-dimensional image using the flat Fresnel lens, a globe-shaped boundary surface 225 can be formed on a screen. Here, the viewer can only observe the 3-dimensional image in the globe-shaped boundary surface 225.
In FIG. 2, a viewer 240a at the center portion among viewers 240a, 240b and 240c can sense a 3-dimensional image 230a with a certain degree of depth. However, the viewers 240b and 240c see images 230b and 230c that are distorted and appear to be smaller than the image 230a viewed by the viewer 240a, and the images 230b and 230c appear to be curved into the screen 210 due to the reduced sense of depth. Therefore, the 3-dimensional images can be distorted according to the viewers' positions. Also, due to narrow viewing angles, images cannot be observed by multiple viewers at the same time and can be viewed only within certain portions.
As shown in FIG. 2, according to the positions of the viewers 240a, 240b and 240c, the 3-dimensional effect of 3-dimensional images can be varied. From the center portion, the left and right image display areas, i.e. viewing angles, can be very narrow, so that the center portion of the entire screen 210 can be only used when rendering the 3-dimensional images. So, due to the limits of the viewing angles and the display area of the 3-dimensional images in prior art, this method may not be applicable in various sectors. In this way, the output image focal plane 125 formed in a spherical shape in accordance with prior art has narrow viewing angles while the distortion of images becomes intensified from the center of the image source to the edges according to a 2-dimensional plane image of the input image source rendered in a spherical shape. When embodying the methods of prior art, the rendition of images can be limited due to the limited use of ¼ of the center portion against the total screen area when rendering the 3-dimensional images, and the distortion of image becomes intensified at the boundary plane on which the 3-dimensional images are rendered, so that the narrow viewing angles are inevitable when using the methods.